Screening Outdoor Compressors Can Hurt Performance and Equipment

Field tests show that the unrestricted airflow is more important than shade.compressor with shrub.jpg

You can't blame people for thinking it might be a good idea to shield outdoor air conditioner compressors from the blazing heat of the sun. It's an important consideration, because more than ever, compressors for electrical mini-splits are being installed on the roof, where they are exposed to full sun all day long, often sitting just above a black, roasting EPDM roof covering.

I became curious about shading outdoor compressors when installing one recently. Among the many choices I had to make was where to locate it, whether or not to screen it with a fence or shrubbery, and whether to try to shade it from the sun to improve performance.

Easy on the Shrubs and Trees

But the research shows that it's nowhere near that simple. Yes, shading a compressor can theoretically give a bump in efficiency, up to about 10% in ideal situations. But those conditions are hard to achieve. The culprit most often, is that the units simply move too much air, too fast, to play well with shrubberies and fences.

"Outdoor AC units draw in a volume of air that greatly exceeds that of the nearby shaded air volume.  Air-cooled  condensers  move  600 - 1200  cfm  of  air  per  ton  (80-160 L/s/kWt) of cooling capacity (ASHRAE, 1992). For  instance, a typical 3-ton (10.6 kWt) air conditioner's 300-W condenser fan would draw 2,800 cfm (1321 L/s) of air at a very low static pressure across the coil  (Proctor  et  al.,1994). Thus, the unit would process 168,000 cubic feet (4.76  x  106L) of air per hour. Assuming no mixing, this would represent a volumetric equivalent to a cube of air with sides measuring 55 feet (16.8 m)."

The problem with natural shading in the form of shrubs or trees is typically proximity. The area of shade has to be very large in order to meet the high airflow demands of the unit. And if that zone is not big enough, the shading can actually backfire, and cause an increase in the unit's energy use. The same article notes that one of the test homes recorded a 15% increase in energy consumption due to shrubs actually trapping hot air. Exhaust air and intake air became mixed, making the machine work harder to achieve the same end result.

The researchers ultimately concluded that localized compressor shading generally results in unit energy savings of less than 3 percent. They suggest putting the units on the north side of the building, with no shrubs or trees nearby, and calling it good.

Trellis and Baffle Effect

Another area that needs additional research is the impact of fences, walls and even proximity to adjacent buildings on compressor performance. For my installation, to appease a nearby neighbor, I installed baffles on the Mitsubishi unit, which apparently is rarely done on residential compressors. I asked the technicians whether the baffles would impede airflow (and thus affect performance). He checked with the manufacturer, who asserts that these baffles are used in data centers, and do not affect performance, but I'd like to see more third-party analysis.

trellis screening.jpg

If trees and shrubs reduce performance, then it's likely fences and trellises do as well. Consumer publications sometimes advocate putting a trellis above an exposed unit, to reduce how much direct solar heating it has to adjust for. This use was examined in the study of shrubs and trees, and found to have a small positive impact on performance. But what about trellises and fences used cosmetically to appease historic preservationists, NIMBYs or homeowner associations? In many cases, the owner may be negatively impacting both the performance and the durability of compressor equipment by making in work harder to produce the same results.

Shade the House, Not the Compressor

A home with tree canopy has demonstrably better energy performance (in hot climates) than a treeless one, and that may be the best use of landscaping for a home with mini-splits and a compressor. A Texas study of more than 100 homes back in the 1990s found a reduction of 11%- to 27% in energy use between homes with no shade, and heavily tree-shaded alternatives.